If two workpieces could be effectively welded together by the friction welding with a certain particular phase angle therebetween, the application range might be increased to a large extent because finished workpieces could be friction welded at once to a final finished product. It has been a long-dreamed technique at least in the conventional frictional welding where two metal pieces are joined by heat generated from relative rotation and a few prior art methods are published, e.g., as Japanese patents, Tokuko-Sho-No. 47-3523, Tokuko-Sho-No. 48-17421 etc. (U.S. Pat. No. 3,705,678 is also published but this is not of a relative rotation type), however, neither of these inventions has been utilized successfully to develop commercial apparatus seemingly due to their inherent defects. In case of the No. 47-3523 invention the phase angle adjustment takes place just after the relative rotation once stops to a standstill; therefore, a slow speed twisting motion is applied again on the workpieces once welded together to adjust phase angle and as the result, the physical properties of the welded part is apt to be much injured and/or to vary in a wide range.
In the case of No. 48-17421 invention the phase angle adjustment is carried out by stopping the relative rotation at a time when at least 360.degree. angle of the rotation is still left enough to complete the phase adjustment before the rotation finally stops, and the speed at the time usually ranges 300 to 700 rpm. Therefore, instant stopping from such a high rotation speed incurs tremendous shock and noise resulting in short life of apparatus and prevention of commercial use.